In the field of radiation therapy, high energy radiation is used to deliver a therapeutic dose to targets within the body to treat cancer. The machines that generate and deliver the radiation produce beams from a source that rotates around the patient. Stray radiation from several sources must be shielded by a thick-walled room or bunker so as to not affect nearby workers and the public. The three main sources of stray radiation that must be shielded are stray radiation from the radiation source and associated beam-shaping elements, scatter from the patient, and the remainder of the beam that travels through the patient.
The shielding of the radiation source is done within the machine, but a significant fraction still escapes. The leakage radiation from a standard linear accelerator must be below 0.1% to meet regulatory requirements. This is still too intense to allow out of the treatment room.
Scatter from the patient as the beam transits the tissue before, including and after the target volume is also significant, but of lower effective energy. This source is also considered separately in calculating the shielding requirements of the bunker.
The most intense source of stray radiation during a treatment is the direct treatment beam that exits the patient after delivering dose to the path of the radiation in the body. Some treatment machines have been supplied with a beam stop that absorbs a large fraction of this direct beam. The beam stop is often used as a counterweight for the shielded rotating radiation source. It is generally designed to absorb as much of the beam as possible given the weight required to balance the radiation source. Scatter from the beam stop also contributes to stray radiation within the treatment room. A typical thickness of lead, for example required to attenuate a high energy photon beam by a factor of ten (a tenth-value layer or TVL) is 5.7 cm. An 18 cm thick lead beam stop reduces the beam intensity by a factor of 1000. The resulting transmission through this beam stop still exceeds the dose rate limit for occupational exposure given a typical workload for the therapy machine.
The bunker that contains the treatment machine is generally a distinctly different part of the whole installation. It is designed to house the treatment machine by trained medical physicists certified in shielding design, and built as an architectural structure of the building, usually underground or at ground level. The bunker walls are typically 2-3.5 meters thick and made of concrete. The additional cost of the bunker constitutes a significant fraction of the total installed cost of the system, and requires a large footprint in the building to accommodate the treatment machine and bunker. This arrangement of radiation source and bunker is a result of existing machines using an open beam to be able to treat any area of the body. The typical radiation therapy system sold today is optimized for treating deep-seated tumors near critical structures.